Method of manufacturing a metal-ceramic circuit board

ABSTRACT

A metal-ceramic circuit board is characterized by being constituted by bonding on a base plate of aluminum or aluminum alloy at least one of ceramic substrate boards having a conductive metal member for an electronic circuit. A method of manufacturing a metal-ceramic circuit board is characterized by comprising the steps of melting aluminum or aluminum alloy in a vacuum or inert gas atmosphere to form a molten metal, contacting one surface of a ceramic substrate board directly with the molten metal in a vacuum or inert gas atmosphere, cooling the molten metal and the ceramic substrate board to form a base plate of aluminum or aluminum alloy, which is bonded directly on the ceramic substrate board without forming any oxidizing film therebetween and bonding a conductive metal member for an electronic circuit on the ceramic substrate board by using a brazing material. The base plate has a proof stress not higher than 320 (MPa) and a thickness not smaller than 1 mm.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 11/066,912, entitled “Method of Manufacturing a Metal-CeramicCircuit Board,” filed Feb. 25, 2005, now abandoned, which is adivisional of U.S. application Ser. No. 10/242,022, entitled “Method ofManufacturing, a Metal-Ceramic Circuit Board,” filed Sep. 12, 2002, nowU.S. Pat. No. 6,938,333, which is a divisional of U.S. application Ser.No. 09/848,140, entitled “Metal-Ceramic Circuit Board,” filed on May 3,2001 now U.S. Pat. No. 7,348,493.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing ametal-ceramic circuit board, and, more particularly, relates to ametal-ceramic circuit substrate board having a high heat-cycleresistance, which is suitable for the installation of high powerelectronic parts, such as power modules, and a manufacturing methodthereof.

BACKGROUND OF THE INVENTION Description of the Prior Art

Recently, high power modules have been used to control a large electriccurrent of electric automobiles, electric cars, tooling machines or thelike. The power modules have mainly a plurality of semiconductor tips. Ahigh electric insulation performance is required for a substrate boardto which the semiconductor tips are fixed, in order to obtain a largecurrent from a front surface and a back surface of each semiconductortip. Further, a temperature of the semiconductor tip is elevated by heatgenerated when a large current is passed through the semiconductor tip.Accordingly, good heat conductivity has been required for the substrateboard on which the semiconductor tips are fixed, and parts surroundingthe board.

FIG. 5 shows a conventional power module in section. The power modulehas semiconductor tips 1, brazing material layers 4, a metal layer 3, aceramic substrate board 2 as an insulating substrate board, a metallayer 5, a brazing material layer 6, and a metal base plate 7 piled inthis order. Reference numeral 8 denotes plating layers formed on themetal layers 3 and 5, and the metal base plate 7. Wirings between thesemiconductor tips 1 are omitted in FIG. 5.

Heretofore, various methods have been proposed to bond an aluminum plateand a ceramic substrate board as shown in Japanese Unexamined UtilityModel Publication No. 57945/1991 and Japanese Unexamined Utility ModelPublication No. 68448/1990. Among these methods, an aluminum plate isbonded to an aluminum nitride board or an alumina board by using abrazing material of Al—Si series or Al—Ge series. U.S. Pat. No.3,994,430, published on 1976, shows the use of silicone as an aluminumbinding assistant.

However, such conventional power modules have following problems becausethe ceramic substrate board 2 is fixed to the metal base plate 7 throughthe metal layer 5 and the brazing material layer 6.

(1) The construction of the power module is complicated because betweenthe ceramic substrate board 2 and the metal base plate 7, the metallayer 5, the plating layer 8, the brazing material layer 6 and theplating layer 8 are arranged in this order. Accordingly, each of thecomponents is heated and cooled repeatedly due to the repetition of thestart and stop of the electrical conduction, so that cracks aregenerated on the contacting surfaces of the components depending on thedifference in thermal expansion coefficient between the components.

(2) The heat conductivity and the heat radiation ability are reducedbecause the brazing material layer 6 exists between the ceramicsubstrate board 2 and the metal base plate 7.

(3) The lead brazing material has been used in spite of the fact thatthe maker of the electric parts wants to reduce the quantity of use ofthe lead brazing material.

(4) The surface treatment such as plating or brazing is required inorder to improve the adhesivity of the brazing material layer 6 to theceramic substrate board 2 and the metal base plate 7, so that the costbecomes high.

(5) A copper base plate has been used as a metal base plate. However,the thermal expansion coefficient of the copper is larger than that ofthe ceramics. Accordingly, cracks are formed easily in the ceramics at aportion where the ceramics is contacted with the copper base plate andthe reliability of the power modules becomes low when the heat and coolare repeated. Further, the base plate such as a copper molybdenum alloyor aluminum silicon carbide complex is low in thermal conductivity andhigh in cost.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of making aceramic-metal composite member of a various form having an excellentproperty, which is formed by bounding directly a ceramic substrate boardwith an aluminum base plate.

Further object of the present invention is to obtain a method ofmanufacturing a ceramic-metal composite member with low cost in massproduction basis.

The present inventors have made various studies and found that the abovetask can be solved by using aluminum or aluminum alloy as a material ofa base plate, and bonding a ceramics with the base plate of aluminum oraluminum alloy of which thickness is larger than a predetermined valueand of which proof stress is lower than a predetermined value, in such amanner that the aluminum or aluminum alloy is molten in a mold andcooling it by contacting with the ceramics.

Specifically, the ceramics is broken or the base plate is warped on alarge scale if a hard metal is used as a base plate, or the base plateis bonded to the ceramics by brazing, without using a soldering methodof low temperature. The present inventors have made various studies andfurther found that the above defect can be obviated by bonding theceramics directly to an aluminum or aluminum alloy plate having verysmall proof stress without using any brazing material. The mechanism ofthe above action is not clear, however, the inventors assume that theresidual stress formed by the difference in thermal expansioncoefficient between the aluminum or aluminum alloy plate of low proofstress and the ceramics when they are bonded is absorbed by thedeformations etc. of the aluminum or aluminum alloy.

A metal-ceramic circuit board of the present invention is characterizedby comprising a base plate of aluminum or aluminum alloy and a ceramicsubstrate board, wherein one surface of the ceramic substrate board isbonded directly to the base plate, and the base plate has a proof stressnot higher than 320 (MPa) and a thickness not smaller than 1 mm.

A power module of the present invention is characterized by comprising abase plate of aluminum or aluminum alloy, a ceramic substrate board, anda semiconductor tip wherein one surface of the ceramic substrate boardis bonded directly to the base plate, said semiconductor tip is providedon the other surface of said ceramic substrate board and the base platehas a proof stress not higher than 320 (MPa) and a thickness not smallerthan 1 mm.

A method of manufacturing a metal-ceramic circuit board of the presentinvention is characterized by comprising the steps of melting aluminumor aluminum alloy in a vacuum or inert gas atmosphere to from a moltenmetal, contacting one surface of a ceramic substrate board directly withsaid molten metal in a vacuum or inert gas atmosphere, and cooling saidmolten metal and said ceramic substrate board to form a base plate ofaluminum or aluminum alloy, which is bonded directly on said one surfaceof the ceramic substrate board.

A method of manufacturing a metal-ceramic circuit board of the presentinvention is characterized by comprising the steps of melting aluminumor aluminum alloy in a vacuum or inert gas atmosphere to from a moltenmetal, contacting one surface of a ceramic substrate board directly withsaid molten metal in a vacuum or inert gas atmosphere, cooling saidmolten metal and said ceramic substrate board to form a base plate ofaluminum or aluminum alloy, which is bonded directly on said one surfaceof the ceramic substrate board, and bonding a conductive metal memberfor an electronic circuit on the other surface of said ceramic substrateboard by using a brazing material.

A method of manufacturing a metal-ceramic circuit board of the presentinvention is characterized by comprising the steps of melting aluminumor aluminum alloy in a vacuum or inert gas atmosphere to form a moltenmetal, contacting directly one surface of a ceramic substrate board, onthe other surface of which a conductive metal member for an electroniccircuit being bonded by using or without using a brazing material, withsaid molten metal in a vacuum or inert gas atmosphere, and cooling saidmolten metal and said ceramic substrate board to form a base plate ofaluminum or aluminum alloy, which is bonded directly on said one surfaceof the ceramic substrate board.

A method of manufacturing a power module of the present invention ischaracterized by comprising the steps of melting aluminum or aluminumalloy in a vacuum or inert gas atmosphere to form a molten metal,contacting one surface of a ceramic substrate board directly with saidmolten metal in a vacuum or inert gas atmosphere, cooling said moltenmetal and said ceramic substrate board to form a base plate of aluminumor aluminum alloy, which is bonded directly on said one surface of theceramic substrate board, forming a metal layer of desired pattern on theother surface of said ceramic substrate board, and fixing asemiconductor tip on said metal layer.

The conductive metal member is characterized by containing at least onemetal selected from copper, copper alloy, aluminum, and aluminum alloy.As the base plate, aluminum or aluminum alloy can be used. However,aluminum is best, because it has a high heat conductivity, a highheat-cycle resistance, and is low in melting point and easy tomanufacture.

As the conductive metal member, copper or copper alloy, aluminum oraluminum alloy is suitable, in case that a special high conductivity isnecessary, or it is sufficient to withstand the thermal cycle test of1000 times.

It is preferable to use aluminum or aluminum alloy in case that it isnecessary to withstand the thermal cycle test of more than 3000 times.

The Au plating or Ni plating can be formed on said metal member in orderto improve the adhesivity of the brazing material layer to the metalmember, and the corrosion resistance.

Said ceramic substrate board is made of a material selected fromalumina, aluminum nitride and silicon nitride.

As said ceramic substrate board, alumina is preferable, because it has ahigh insulating property, a high flexibility of use such that thecircuit can be manufactured by the direct bonding of copper, and ischeap. The aluminum nitride has a high thermal conductivity and a highheat radiation ability, so that the tip for controlling a large electriccurrent can be installed. The silicon nitride has a high proof stressand a high heat-cycle resistance, so that it can be used in the strictcircumstances, such as in the engine room.

Said base plate is use for enhancing the mechanical proof stress and theheat radiation ability of the module. The wording of the direct bondingmeans that the base plate is bonded on the ceramic substrate board so asto have a necessary proof stress without using any binding assistantsuch a brazing material or the like.

These and other aspects and objects of the present invention will bebetter appreciated and understood when considered in conjunction withthe following description and the accompanying drawings. It should beunderstood, however, that the following description, while indicatingpreferred embodiments of the present invention, is given by way ofillustration and not of limitation. Many changes and modifications maybe made within the scope of the present invention without departing fromthe spirit thereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertically sectioned front view of a furnace for explaininga principle of the present invention.

FIG. 2 is a vertically sectioned front view of a power module on anembodiment in accordance with the present invention.

FIG. 3 is a vertically sectioned front view of a furnace for explaininganother embodiment of the present invention.

FIG. 4 is a vertically sectioned front view of a furnace for explainingthe other embodiment of the present invention.

FIG. 5 is a vertically sectioned front view of a conventional powermodule.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following descriptions, parts of the power module of the presentinvention which are similar to corresponding parts of the power moduleshown in FIG. 5 have been given corresponding reference numerals andneed not be further re-described.

EXAMPLE 1

In a first Example of the present invention, aluminum of 99.99% inpurity was set in a crucible formed at an upper portion of a furnace 9,and a plurality of ceramic substrate boards 2 of aluminum nitride wereset on an inside bottom portion of the furnace 9 below the crucible. Thecrucible was closed by a piston 10 and the furnace 9 was filled withnitrogen gas. Then, the furnace 9 was heated at 750° C. by a heater 11to melt the aluminum in the crucible. The molten aluminum 13 was pushedout by the piston 10 through a narrow conduit 12 connecting between acenter bottom portion of the crucible and the inside bottom portion ofthe furnace 9, so that the molten aluminum 13 was poured on the ceramicsubstrate boards 2 until the height of the molten aluminum 13 on theceramic substrate boards 2 reached a predetermined value. Then, themolten aluminum 13 on the ceramic substrate boards 2 was cooled andsolidified gradually, to form an aluminum base plate 7 bonded directlyon the ceramic substrate boards 2. Thus obtained aluminum base plate 7had a thickness of 5 mm and a proof stress of 40 MPa. The value of theproof stress was measured along JIS Z2241 a test piece of JIS Z2201.

Then, the base plate 7 with the ceramic substrate boards 2 was taken outfrom the furnace 9 in order to form a circuit portion on the ceramicsubstrate board 2. A desired pattern of a brazing material consisting ofAl in an amount of 87.5% by weight and Si in an amount of 12.5% byweight (not shown) was printed by using a screen printer, and dried at80° C. An aluminum rolled plate of a desired pattern was placed as ametal layer 3 on the dried brazing material, and heated at 575° C. in avacuum furnace. Then, an electro-less nickel plating layer 8 was formedon the metal layer 3 and a semiconductor tip 1 was fixed on the metallayer 3 through the plating layer 8 and a brazing material layer 4 toform a power module as shown in FIG. 2.

A thermal cycle test was performed to evaluate the power module. Afterthe thermal cycle of 4000 times, no change was recognized on theboundary surface between the ceramic substrate layer 2 and the baseplate 7.

EXAMPLE 2

A power module having a metal-ceramic circuit substrate board as shownin FIG. 2 was formed under the same conditions as in the Example 1except that the thickness of the aluminum base plate 7 was change from 5mm to 1 mm. A thermal cycle test was performed to evaluate the powermodule. After the thermal cycle of 4000 times, no change was recognizedon the boundary surface between the ceramic substrate layer 2 and thebase plate 7 similar to the Example 1.

EXAMPLE 3

A power module having a metal-ceramic circuit substrate board as shownin FIG. 2 was formed under the same conditions as in the Example 1except that the thickness of the aluminum base plate was change from 5mm to 10 mm. A thermal cycle test was performed to evaluate the powermodule. After the thermal cycle of 3000 times, no change was recognizedon the boundary surface between the ceramic substrate layer 2 and thebase plate 7.

EXAMPLE 4

A power module having a metal-ceramic circuit substrate board as shownin FIG. 2 was formed under the same conditions as in the Example 1except that the thickness of the aluminum base plate was change from 5mm to 30 mm. A thermal cycle test was performed to evaluate the powermodule. After the thermal cycle of 3000 times, no change was recognizedon the boundary surface between the ceramic substrate layer 2 and thebase plate 7 similar to the Example 3.

EXAMPLE 5

A power module having a metal-ceramic circuit substrate board as shownin FIG. 2 was formed under the same conditions as in the Example 1except that the material of the base plate 7 was changed from aluminumof 99.99% in purity to aluminum alloy consisting of Al in an amount of95.5% by weight and Cu in an amount of 4.5% by weight.

The base plate 7 had a thickness of 5 mm and a proof stress of 95 MPa.

A thermal cycle test was performed to evaluate the power module. Afterthe thermal cycle of 3000 times, no change was recognized on theboundary surface between the ceramic substrate layer 2 and the baseplate 7 similar to the Example 3.

EXAMPLE 6

A power module having a metal-ceramic circuit substrate board as shownin FIG. 2 was formed under the same conditions as in the Example 1except that the material of the base plate 7 is changed from aluminum of99.99% in purity to aluminum alloy consisting of Al in an amount of87.5% by weight and Si in an amount of 12.5% by weight.

The base plate 7 had a thickness of 5 mm and a proof stress of 320 MPa.

A thermal cycle test was performed to evaluate the power module. Afterthe thermal cycle of 3000 times, no change was recognized on theboundary surface between the ceramic substrate layer 2 and the baseplate 7 similar to the Example 3.

EXAMPLE 7

A power module having a metal-ceramic circuit substrate board as shownin FIG. 2 was formed under the same conditions as in the Example 1except that the material of the ceramic substrate board 2 is changedfrom aluminum nitride to silicone nitride.

A thermal cycle test was performed to evaluate the power module. Afterthe thermal cycle of 4000 times, no change was recognized on theboundary surface between the ceramic substrate layer 2 and the baseplate 7 similar to the Example 1.

EXAMPLE 8

A power module having a metal-ceramic circuit substrate board as shownin FIG. 2 was formed under the same conditions as in the Example 1except that fins were provided on the base plate 7 in order to improvethe heat radiation.

A thermal cycle test was performed to evaluate the power module. Afterthe thermal cycle of 4000 times, no change was recognized on theboundary surface between the ceramic substrate layer 2 and the baseplate 7 similar to the Example 1.

EXAMPLE 9

In order to form a circuit portion on an upper surface of the ceramicsubstrate board 2 of aluminum nitride, an actuated metal brazingmaterial consisting of Ag in an amount of 90% by weight, Ti in an amountof 5% by weight and Cu in an amount of 5% by weight was printed by usinga screen printer, and dried at 80° C. A copper rolled plate was placedas a metal layer 3 on the dried brazing material, and heated at 800° C.in a vacuum furnace, so that the metal layer 3 was bonded on the ceramicsubstrate board 2. Then, an etching resist was printed on the copperportion by using the screen printer, UV dried and subjected to etchingusing a ferric chloride solution to form a desired pattern 14. Theceramic substrate boards 2 with the metal layers 3 were placed on aninside bottom portion of a furnace 9 with a bottom surface of theceramic substrate board 2 facing upward as shown in FIG. 3. Aluminum of99.99% in purity was set in a crucible formed at an upper portion of thefurnace 9, and the crucible was closed by a piston 10 and the furnace 9was filled with nitrogen gas. Then, the furnace 9 was heated at 750° C.by a heater 11 to melt the aluminum in the crucible. The molten aluminum13 was pushed out by the piston 10 through a narrow conduit 12connecting between a center bottom portion of the crucible and theinside bottom portion of the furnace 9, so that the molten aluminum 13was poured on the ceramic substrate boards 2 until the height of themolten aluminum 13 on the ceramic substrate boards 2 reached apredetermined value. Then, the molten aluminum 13 on the ceramicsubstrate boards 2 was cooled and solidified gradually to form analuminum base plate 7 bonded directly on the bottom surface of theceramic substrate boards 2. Thus obtained aluminum base plate 7 had athickness of 5 mm and a proof stress of 40 MPa.

Then, the base plate 7 with the ceramic substrate boards 2 and the metallayers 3 was taken out from the furnace 9 and a semiconductor tip 1 wasfixed on the metal layer 3 through a brazing material layer 4 to form apower module as shown in FIG. 2.

A thermal cycle test was performed to evaluate the power module. Afterthe thermal cycle of 4000 times, no change was recognized on theboundary surface between the ceramic substrate layer 2 and the baseplate 7.

EXAMPLE 10

A plurality of ceramic substrate boards 2 of aluminum nitride werebonded on a base plate 7 of aluminum under the same conditions as in theExample 1. As shown in FIG. 4, a furnace 15 is used and aluminum of99.99% in purity was set in a crucible formed at an upper portion of thefurnace 15. A plurality of ceramic substrate boards 2 of aluminumnitride bonded on the base plate 7 were set on an inside bottom portionof the furnace 15 below the crucible with the ceramic substrate board 2facing upward. A mold 18 having a mortise of a desired circuit patternwas placed on each ceramic substrate board 2. The crucible was closed bya piston 10 and the furnace 15 was filled with nitrogen gas. Then, thefurnace 15 was heated at 750° C. by a heater 11 to melt the aluminum inthe crucible. The molten aluminum 13 was pushed out by the piston 10through a narrow conduit 16 and narrow conduits 17 a, 17 b and 17 cconnecting between a center bottom portion of the crucible and the molds18. A heat sink 19 was arranged at under side of the base plate 7 inorder to cool and protect the base plate 7. The pushed out moltenaluminum 13 was poured on the ceramic substrate board 2 in the mold 18until the height of the molten aluminum 13 on the ceramic substrateboard 2 reached a predetermined value. Then, the molten aluminum 13 onthe ceramic substrate board 2 was cooled and solidified gradually toform a metal layer 3 on the ceramic substrate board 2. Thus obtainedaluminum base plate 7 had a thickness of 5 mm and a proof stress of 40MPa.

The base plate 7 with the ceramic substrate boards 2 and the metallayers 3 was taken out from the furnace 15. A semiconductor tip 1 wasfixed on the metal layer 3 through a brazing material layer 4 to form apower module as shown in FIG. 2.

A thermal cycle test was performed to evaluate the power module. Afterthe thermal cycle of 4000 times, no change was recognized on theboundary surface between the ceramic substrate layer 2 and the baseplate 7 similar to the Example 1.

COMPARATIVE EXAMPLE 1

A following sample was prepared for comparison. In order to form acircuit portion on an upper surface of a ceramic substrate board ofaluminum nitride, a brazing material consisting of Al in an amount of87.5% by weight and Si in an amount of 12.5% by weight was printed onthe upper surface of the ceramic substrate board 2 by using a screenprinter to form a desired pattern, and dried at 80° C. An aluminumrolled plate of a desired pattern was placed on the brazing material.The same brazing material was printed entirely on a lower surface of theceramic substrate board, an aluminum rolled plate of a desired patternwas placed thereon, and heated at 575° C. in a vacuum furnace. Then, theceramic substrate board was subjected to an electro-less nickel plating.Three sheets of the ceramic substrate board thus obtained were fixed bybrazing on an aluminum base plate having a thickness of 5 mm and apurity of 99.99% which was subjected to an electro-less nickel plating.A semiconductor tip was fixed on the aluminum layer formed on theceramic substrate board to form a power module as shown in FIG. 5.

A thermal cycle test was performed to evaluate the power module, as likeas the Examples. After the thermal cycle of 1000 times, cracks wererecognized in the brazing material layer on the boundary surface betweenthe ceramic substrate board and the base plate.

COMPARATIVE EXAMPLE 2

A following sample was prepared for comparison. A power module as shownin FIG. 5 was formed under the similar manner as in the comparativeExample 1 except that the material of the base plate was changed fromaluminum to a copper molybdenum alloy of 5 mm in thickness. A thermalcycle test was performed to evaluate the power module, as like as theExamples. After the thermal cycle of 3000 times, cracks were recognizedin the brazing material layer on the boundary surface between theceramic substrate layer 2 and the base plate 7.

COMPARATIVE EXAMPLE 3

A following sample was prepared for comparison. A molten aluminum asshown in the Example 1 was contacted directly to both surfaces of aceramic substrate board of aluminum nitride, cooled and solidified toform aluminum layers. Then, in order to form a circuit portion on one ofthe both surfaces of the ceramic substrate board, an etching resist wasprinted on the one surface by using a screen printer, UV dried andsubjected to etching using a ferric chloride solution to form a desiredcircuit pattern. The ceramic substrate board with the circuit patternwas subjected to an electro-less nickel plating. Three sheets of theceramic substrate board thus obtained were fixed by brazing on analuminum base plate subjected to an electro-less nickel plating andhaving a thickness of 5 mm and a purity of 99.99%. Further, asemiconductor tip was provided on each of the substrate boards to form apower module as shown in FIG. 5.

A thermal cycle test was performed to evaluate the power module, as likeas the Examples. After the thermal cycle of 3000 times, cracks wererecognized in the brazing material layer on the boundary surface betweenthe ceramic substrate board and the base plate.

COMPARATIVE EXAMPLE 4

A following sample was prepared for comparison. In order to bond threesheets of ceramic substrate board of aluminum nitride on one surface ofa base plate of 99.99% in purity having a thickness of 5 mm, a brazingmaterial consisting of Al in an amount of 87.5% by weight and Si in anamount of 12.5% by weight was printed on the base plate by using ascreen printer and dried at 80° C. A ceramic substrate board was placedon the brazing material and heated at 575° C. in a vacuum furnace. Itwas examined to form a circuit on the other side of the base plate by abrazing method similar to the above, however all of the ceramicsubstrate boards were cracked when the ceramic substrate boards werebonded on the base plate.

COMPARATIVE EXAMPLE 5

A following sample was prepared for comparison. It was examined to forma power module having a metal-ceramic circuit substrate board as shownin FIG. 2 under the same conditions as in the Example 1 except that thethickness of the aluminum base plate was change from 5 mm to 0.5 mm.However, the base plate deformed easily because of the lack of proofstress.

COMPARATIVE EXAMPLE 6

A following sample was prepared for comparison. It was examined to forma power module having a metal-ceramic circuit substrate board as shownin FIG. 2 under the same conditions as in the Example 1 except that thematerial of the base plate was changed from aluminum of 99.99% in purityto aluminum alloy consisting of Al in an amount of 88% by weight, Cu inan amount of 2% by weight, Mg in an amount of 3% by weight and Zn in anamount of 7% by weight. The base plate had a thickness of 5 mm and aproof stress of 540 MPa.

However, all of the ceramic substrate boards are cracked when theceramic substrate boards were bonded on the base plate.

The above results are shown in Table 1 and Table 2.

TABLE 1 Base plate Circuit Manufacturing method Strength ThicknessInsulating material Circuit Bonding method Bonding method Number Basemetal (MPa) (mm) Ceramics metal of base side Order of circuit sideExample 1 99.99% Al 10 5 AIN Al Hot-dip method → Brazing method Example2 99.99% Al 40 1 AIN Al Hot-dip method → Brazing method Example 3 99.99%Al 40 10 AIN Al Hot-dip method → Brazing method Example 4 99.99% Al 4030 AIN Al Hot-dip method → Brazing method Example 5 Al + Cu 95 5 AIN AlHot-dip method → Brazing method Example 6 Al + Si 320 5 AIN Al Hot-dipmethod → Brazing method Example 7 99.99% Al 40 5 Si₃N₄ Al Hot-dip method→ Brazing method Example 8 99.99% Al 40 5 AIN Al Hot-dip method →Brazing method Example 9 99.99% Al 40 5 AIN Cu Hot-dip method

Brazing method Example 10 99.99% Al 40 5 AIN Al Hot-dip method → Hot-dipmethod Comparative 99.99% Al 40 5 AIN Al Brazing

Brazing method Example 1 Comparative Cu + Mo 40 5 AIN Al Brazing

Brazing method Example 2 Comparative 99.99% Al 40 5 AIN Al Brazing

Hot-dip method Example 3 Comparative 99.99% Al 40 5 AIN Al Brazingmethod → Example 4 Comparative 99.99% Al 40 0.5 AIN Al Hot-dip method →Brazing method Example 5 Comparative Al + Cu + Mg + Zn 540 5 AIN AlHot-dip method → Brazing method Example 6

TABLE 2 Property Crack at bond- Module Number Others Heat cycle strengthing strength Example 1 No problem at 4000 cycle No No problem Example 2No problem at 4000 cycle No No problem Example 3 No problem at 3000cycle No No problem Example 4 No problem at 3000 cycle No No problemExample 5 No problem at 3000 cycle No No problem Example 6 No problem at3000 cycle No No problem Example 7 No problem at 4000 cycle No Noproblem Example 8 With fin No problem at 4000 cycle No No problemExample 9 No problem at 4000 cycle No No problem Example 10 No problemat 4000 cycle No No problem. Comparative Crack at 1000 cycle No Noproblem Example 1 Comparative Crack at 3000 cycle No No problem Example2 Comparative Crack at 3000 cycle No No problem Example 3 ComparativeYes No problem Example 4 Comparative Yes Problem Example 5 ComparativeYes No problem Example 6

According to the present invention, following advantages can beobtained.

(1) The reliability of the metal-ceramic circuit board when the coolingand heating are repeated, can be elevated remarkably, because thestructure between the ceramic substrate board and the base plate issimple. Specifically, aluminum or aluminum alloy is used as the materialof the base plate and bonded directly on the ceramic substrate board, sothat any irregularity in thermal expansion and contraction of the baseplate when it is heated and cooled is eliminated, and the crack isprevented from being occurred in the contact portion of the ceramicsubstrate board by the softness of aluminum.

(2) A high heat conductivity can be obtained because the structurebetween the ceramic substrate board and the base plate is simple, andthe brazing material layer of low in heat conductivity can be omitted.

(3) The cost can be reduced because the structure between the ceramicsubstrate board and the base plate is simple, so that any brazing forbonding the both can be omitted, and that any surface treatment such asplating or the brazing can be omitted.

(4) Copper used conventionally as a base plate is cheap. However, thethermal expansion coefficient is larger than that of the ceramics, sothat the reliability is low because a crack is formed easily on thebonding surface between the ceramic substrate board and the base platewhen the heating and cooling are repeated. Copper molybdenum alloy oraluminum silicon carbide composite material is low in heat conductivityand high in cost. On the contrary, aluminum is cheap and very small inproof stress, though the thermal expansion coefficient is high, so thatthe crack is hardly formed on the boundary surface between the ceramicsubstrate board and the base plate even if the heating and cooling arerepeated and that high reliability can be obtained.

(5) It is considered such a manufacturing method that a circuitsubstrate board is manufactured by bonding a base plate of aluminum,aluminum alloy, copper, copper molybdenum alloy, or aluminum siliconcarbide composite material on a ceramic substrate board by using brazingmaterial. However, the ceramic substrate board is deformed to a largeextent and cracks are formed easily in the ceramic substrate board dueto the difference in thermal expansion and reduction between the bondedbase plate and the ceramic substrate board, because the thickness of thebase plate is larger than the thickness of the ceramic substrate board,the bonding layer of brazing material low in flexibility is formedbetween the base plate and the ceramic substrate board. On the contrary,in the present invention, the above defects can be obviated, because thebase plate of aluminum or aluminum alloy of less than 320 (MPa) in proofstress and more than 1 mm in thickness is bonded directly to the ceramicsubstrate board so as to have a bonding portion very high inflexibility.

(6) The substrate board for the power module according to the presentinvention is suitable especially to control a large electric current ofelectric automobiles, electric cars, tooling machines or the like,because the reliability, and the yield are high and the cost is low.

(7) The heat treatment is carried out in the inert gas, so that theoxidization of the material is prevented and the good bonding can beachieved. Further, the temperature in the furnace may be set to550°˜850° C.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. method of manufacturing a metal-ceramic circuit board comprising:bonding a conductive metal member for an electric circuit on a firstsurface of a ceramic substrate board; placing a quantity of aluminum oran aluminum alloy in a furnace; placing said ceramic substrate boardbeing bonded to said conductive metal member for the electric circuit inan inside bottom portion of the furnace such that a second surface ofthe ceramic surface board is facing outwardly from said inside bottomportion the second surface being generally opposed to said first surfacehaving the conductive metal members bonded thereto; creating an inertgas atmosphere inside the furnace; melting the aluminum or aluminumalloy to form a molten metal; dispensing the molten metal via a narrowconduit onto said ceramic substrate board such that the molten metal isin direct contact with a second surface of said ceramic substrate board;and cooling said molten metal and said ceramic substrate board beingbonded to said conductive metal member for the electric circuit to forma base plate of aluminum or aluminum alloy, which is bonded directly onthe second surface of the ceramic substrate board.
 2. The method ofmanufacturing the metal-ceramic circuit board according to claim 1,wherein said conductive metal member for the electric circuit is bondedon the first surface of the ceramic substrate board by using a brazingmaterial.
 3. The method of manufacturing the metal-ceramic circuit boardaccording to claim 2, wherein said ceramic substrate board comprises amaterial selected from alumina, aluminum nitride and silicon nitride. 4.The method of manufacturing the metal-ceramic circuit board according toclaim 2, wherein said conductive metal member contains at least onemetal selected from copper, copper alloy, aluminum, and aluminum alloy.5. The method of manufacturing the metal-ceramic circuit board accordingto claim 1, wherein said conductive metal member for the electriccircuit is bonded on the first surface of the ceramic substrate boarddirectly.
 6. The method of manufacturing the metal-ceramic circuit boardaccording to claim 5, wherein said ceramic substrate board comprises amaterial selected from alumina, aluminum nitride and silicon nitride. 7.The method of manufacturing the metal-ceramic circuit board according toclaim 5, wherein said conductive metal member contains at least onemetal selected from copper, copper alloy, aluminum, and aluminum alloy.